Symmetrical relationships between humans and their environment have been referred to as an extension of symmetries in the human geographical system and have drawn great attention. This paper explored the symmetry betw...Symmetrical relationships between humans and their environment have been referred to as an extension of symmetries in the human geographical system and have drawn great attention. This paper explored the symmetry between physical and human systems through fractal analysis of the road and drainage networks in Wuling mountainous area. We found that both the road and drainage networks reflect weak clustering distributions. The evolution of the road network shared a significant self-organizing composition, while the drainage network showed obvious double fraetal characteristics. The geometric fractal dimension of the road network was larger than that of the drainage network. In addition, when assigned a weight relating to hierarchy or length, neither the road network nor drainage network showed a fractal property. These findings indicated that the fractal evolution of the road network shared certain similarities with fractal distribution of the drainage network. The symmetry between the two systems resulted from an interactive process of destroying symmetry at the lower order and reconstructing symmetry at the higher order. The relationships between the fractal dimensions of the rural-urban road network, the drainage network andthe urban system indicated that the development of this area was to achieve the symmetrical isomorphism of physical-human geographical systems.展开更多
The Cambrian explosion has long been a basic research frontier that concerns many scientific fields. Here we discuss the cause-effect links of the Cambrian explosion on the basis of first appearances of animal phyla i...The Cambrian explosion has long been a basic research frontier that concerns many scientific fields. Here we discuss the cause-effect links of the Cambrian explosion on the basis of first appearances of animal phyla in the fossil record, divergence time, environmental changes, Gene Regulatory Networks, and ecological feedbacks. The first appearances of phyla in the fos- sil record are obviously diachronous but relatively abrupt, concentrated in the first three stages of the Cambrian period （541- 514 Ma）. The actual divergence time may be deep or shallow. Since the gene regulatory networks （GRNs） that control the de- velopment of metazoans were in place before the divergence, the establishment of GRNs is necessary but insufficient for the Cambrian explosion. Thus the Cambrian explosion required environmental triggers. Nutrient availability, oxygenation, and change of seawater composition were potential environmental triggers. The nutrient input, e.g., the phosphorus enrichment in the environment, would cause excess primary production, but it is not directly linked with diversity or disparity. Further in- crease of oxygen level and change of seawater composition during the Ediacaran-Cambrian transition were probably crucial environmental factors that caused the Cambrian explosion, but more detailed geochemical data are required. Many researchers prefer that the Cambrian explosion is an ecological phenomenon, that is, the unprecedented ecological success of ruetazoans during the Early Cambrian, but ecological effects need diverse and abundant animals. Therefore, the establishment of the eco- logical complexity among animals, and between animals and environments, is a consequence rather than a cause of the Cam- brian explosion. It is no doubt that positive ecological feedbacks could facilitate the increase of biodiversity. In a word, the Cambrian explosion happened when environmental changes crossed critical thresholds, led to the initial formation of the meta- zoan-doruinated ecosystem through a series of knock-on ecological processes, i.e., ＂ecological snowball＂ effects.展开更多
基金supported by the National Natural Science Foundation of China project (Grant Nos. 41201130, 41101361, and 41371183)
文摘Symmetrical relationships between humans and their environment have been referred to as an extension of symmetries in the human geographical system and have drawn great attention. This paper explored the symmetry between physical and human systems through fractal analysis of the road and drainage networks in Wuling mountainous area. We found that both the road and drainage networks reflect weak clustering distributions. The evolution of the road network shared a significant self-organizing composition, while the drainage network showed obvious double fraetal characteristics. The geometric fractal dimension of the road network was larger than that of the drainage network. In addition, when assigned a weight relating to hierarchy or length, neither the road network nor drainage network showed a fractal property. These findings indicated that the fractal evolution of the road network shared certain similarities with fractal distribution of the drainage network. The symmetry between the two systems resulted from an interactive process of destroying symmetry at the lower order and reconstructing symmetry at the higher order. The relationships between the fractal dimensions of the rural-urban road network, the drainage network andthe urban system indicated that the development of this area was to achieve the symmetrical isomorphism of physical-human geographical systems.
基金supported by National Basic Research Program of China (Grant No. 2013CB835000)National Natural Science Foundation of China (Grant Nos. 40925005, 41272036)+1 种基金the "111 Project" (Grant No. P201102007)the key project from the State Key Laboratory of Continental Dynamics, Northwest University
文摘The Cambrian explosion has long been a basic research frontier that concerns many scientific fields. Here we discuss the cause-effect links of the Cambrian explosion on the basis of first appearances of animal phyla in the fossil record, divergence time, environmental changes, Gene Regulatory Networks, and ecological feedbacks. The first appearances of phyla in the fos- sil record are obviously diachronous but relatively abrupt, concentrated in the first three stages of the Cambrian period （541- 514 Ma）. The actual divergence time may be deep or shallow. Since the gene regulatory networks （GRNs） that control the de- velopment of metazoans were in place before the divergence, the establishment of GRNs is necessary but insufficient for the Cambrian explosion. Thus the Cambrian explosion required environmental triggers. Nutrient availability, oxygenation, and change of seawater composition were potential environmental triggers. The nutrient input, e.g., the phosphorus enrichment in the environment, would cause excess primary production, but it is not directly linked with diversity or disparity. Further in- crease of oxygen level and change of seawater composition during the Ediacaran-Cambrian transition were probably crucial environmental factors that caused the Cambrian explosion, but more detailed geochemical data are required. Many researchers prefer that the Cambrian explosion is an ecological phenomenon, that is, the unprecedented ecological success of ruetazoans during the Early Cambrian, but ecological effects need diverse and abundant animals. Therefore, the establishment of the eco- logical complexity among animals, and between animals and environments, is a consequence rather than a cause of the Cam- brian explosion. It is no doubt that positive ecological feedbacks could facilitate the increase of biodiversity. In a word, the Cambrian explosion happened when environmental changes crossed critical thresholds, led to the initial formation of the meta- zoan-doruinated ecosystem through a series of knock-on ecological processes, i.e., ＂ecological snowball＂ effects.
